1. Field of the Invention
This invention relates to a catalytic converter for purifying gas such as exhaust gas from an automotive internal combustion engine and to a method of producing the same, and more particularly to such a catalytic converter including a metallic carrier which serves also as a heater element for promoting activation of catalyst metal and to a method of producing the same.
2. Description of the Prior Art
A variety of catalytic converters have been used for purifying exhaust gas emitted from an internal combustion engine of an automotive vehicle. It is a recent trend that a catalytic converter including a metallic carrier has been widely used because of being advantageous from view points of thermal capacity and pressure drop characteristics. As is well known, catalyst substances of the catalytic converter cannot exhibit a sufficient catalytic action if the temperature of the catalytic substances has not reached a predetermined activation level. Accordingly, a considerable time is required to cause the catalytic action of the catalytic substances to reach a sufficient level at starting of the engine.
In this regard, it has been proposed to heat the catalytic substances by using a heater. For this purpose, the above-mentioned metallic carrier is constructed of electrically conductive sheet metals in which one of the sheet metals are used as a resistor or heater element. A stainless steel sheet is usually used as the sheet metal from view points of heat resistance and the like. Such a stainless steel sheet is considerably low in resistivity, and therefore a resistance necessary for the heater element cannot be obtained in case that electric current is passed to a generally cylindrical metallic carrier in a diametrical direction or in a longitudinal direction.
In view of the above, it has been proposed that flat and corrugated sheet metals are coated with electrically insulative ceramic layers and then thereby forming a generally cylindrical metallic carrier. Such a metallic carrier is disclosed in U.S. Pat. No. 3,770,389. In this metallic carrier, the spirally formed flat and corrugated sheet metals are electrically insulated from each other throughout the whole length thereof, and therefore a considerable length of the sheet metal as the resistor can be obtained in case that electric current is flown through the central part and the outer peripheral part of one (for example, the corrugated sheet metal) of the flat and corrugated sheet metals).
However, the following difficulties have been encountered in the above-discussed conventional metallic carrier: The flat and corrugated sheet metals must be formed into the generally cylindrical shape by bending and spirally winding the flat and corrugated sheet metals after the flat and corrugated sheet metals are coated respectively with the electrically insulative ceramic layers. Therefore, a large amount of ceramic material is unavoidably peeled off from the ceramic layers during the step of spirally winding the flat and corrugated sheet metals. This tends to make a short circuit in the metallic carrier, thereby lowering the resistance of the sheet metal serving as the heater element. The thus lowered resistance prevent the sheet metal of the metallic carrier from serving as an effective heater element.